The present invention generally relates to an automotive transfer case. More specifically, the present invention relates to an automotive transfer case which includes a ball screw actuated clutch pack to allow selectively actuation of the transfer case.
In four-wheel drive vehicles, a power transfer system is used to transfer drive torque to all four wheels. In many vehicles, a transfer case is interconnected between the first and second drivelines which is equipped with a dog-type mode clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a part-time four-wheel drive mode. When the mode clutch is disengaged, all drive torque is delivered to the primary driveline for establishing a two-wheel drive mode.
In some four-wheel drive vehicles the transfer case is equipped with an electronically-controlled transfer clutch pack in place of the mode clutch. The clutch pack is operable for selectively directing drive torque to the secondary wheels when traction is lost at the primary wheels for establishing an on-demand four-wheel drive mode. Typically, the clutch pack is a multi-plate clutch assembly installed between the primary and secondary output shafts of the transfer case and which is actuated by a power-operated actuator in response to control signals sent from a controller. The vehicle operator is permitted to select between the two-wheel drive mode and the part-time four-wheel drive mode in addition to on-demand four-wheel drive mode. Specifically, when the two-wheel drive mode is selected, the clutch pack is released for delivering all drive torque to the first output shaft. In contrast, when the part-time four-wheel drive mode is selected, an axial force is placed upon the clutch pack such that the clutch pack is fully engaged for effectively locking the second output shaft to the first output shaft.
One way of applying an axial force onto a clutch pack of this type is with a ball ramp. A ball ramp is comprised of a pair of plates having opposing channels formed therein. Within the channels are balls. The height of the balls is such that the plates are not allowed to touch. The channels are ramped such that when one plate rotates relative to the other, the balls will roll up the ramped channels and force the plates apart, thereby producing an axial force.
Over time, the channels within the plates will wear, which means less axial movement of the plates occurs as the balls roll up the ramped surfaces. This reduces the amount of axial force produced by the ball ramp, and affects the performance of the clutch pack. The only way to repair this condition is to reposition or replace the ball ramp. Further, a ball ramp will only move a limited distance away from the clutch pack. Therefore, the clutch pack cannot be relieved further to allow cooling.
Therefore, there is a need for a transfer case having a clutch pack for selectively engaging the primary and secondary drive shafts of the vehicle which includes a device that applies an axial force to the clutch pack and can be tuned to accommodate for wear of the clutch pack and wear within the device, as well as allowing the device to be backed away from the clutch pack to allow the clutch pack to cool.